Dental panoramic X-ray imaging is a well-known dental radiographic procedure. Its purpose is to produce an X-ray image of the entire jaw for diagnosis as opposed to a partial image such as obtained by intra-oral X-ray imaging. Other extra-oral dental X-ray imaging systems include cone beam dental computed tomography (CT) system for 3-D tomosynthetic volumetric reconstruction and transverse slicing. Regular panoramic imaging is used mostly from general purpose orthodontics, while 3-D imaging and transverse slicing maybe used more often for dental implantation.
Dental panoramic X-ray imaging units, cone beam units and transverse slicing units, a.k.a. orthopantomographs (OPGs) or dental CT's, are available with analog film were suitable and with digital sensors (in case of cone beam CT). Digital OPGs currently available in the market utilize sensors based on CCDs coupled to a phosphor or scintillator, and operate in a Time Delay Integration Mode (TDI), or flat panels utilizing a-Si (amorphous silicon) TFT arrays (Thin Film Transistor) with again a scintillator on the top. Both CCD's and TFT flat panels used convert x-rays to light and then light is converted to an electronic signal inside the CCD or TFT. This equipment and especially the cone beam CT and transverse slicing equipment which produce multiple frames, these systems are not fast enough for continuous exposure. Consequently, real time viewing is not possible. A fully equipped dental office needs to have several types of so called “intra-oral” sensors to complete the range of functionalities needed to cover general maxillofacial examinations, fillings and cavities, orthodontics, implantology and surgery. As can be appreciated, this is a cost burden that probably only large clinics can afford.
An orthopantomograph is made up of four functional units: an X-ray generator; an imaging device; a mechanical manipulator; and a user control panel. The purpose of the X-ray generator is to create the x-rays that penetrate the head of the patient and arrive at the imaging device. The X-ray generator or source is able to generate x-rays with different spectra by varying the high voltage level and with differing intensity by varying the current. The imaging device detects and converts the x-rays incident on it into an image. The mechanical manipulator displaces both the imaging device and the X-ray generator in such a way that a proper panoramic image of the plane-of-interest is formed. The user control panel or the user interface, is used to control different settings of the OPG or to initiate and control an X-ray exposure event.
Currently digital imaging devices accomplish their purpose through: absorption by traditional films, or by digital two-stage indirect conversion (using a CCD with a scintillator). Linear arrays of CCD's are used in OPG's but flat panels based on TFT or image intensifiers are used in cone beam dental CT. A typical cone beam dental CT system does not differ in any substantial way from the OPG system, except that the X-ray beam is cone shaped rather than fan shaped. Additionally the cone beam systems require that the X-ray scan be performed for longer times and in step fashion (i.e., not continuously) because Image Intesifiers (IIs) or TFT panels are too slow and lack sensitivity.
Dental Panoramic, Dental Transverse and Dental 3-D X-Ray Imaging
In dental panoramic X-ray imaging, the image is captured during a process in which both the X-ray generator and the imaging device move around the patient's head according to a predetermined geometric path and speed profile. The movement is synchronized in such a way that an image of the pre-determined layer of interest is formed according to the predetermined geometry and speed profile. Because of the shape of the human jaw, this layer is a non-planar structure. It in fact varies with the morphology of each individual's jaw. To simplify the procedure while still maintaining high resolution, a standard shape is used that is applied to all human males, females, and children of certain ages, is used. The exact shape of the layer-of-interest depends on the dental procedure in question; the predetermined geometric path of the source and detector (optionally varying depending on the patient type); and the predetermined speed profile. The layer can usually be adjusted by selecting a different pre-determined, preset program in the OPG by changing the path of movement and/or also the speed profile. Different programs can alter the general parameters of the profile to match the patient (again, e.g., whether child or adult) or to only image a part of the full profile (i.e., front teeth, left/right side etc.). But in each case when a new panoramic layer is needed a new exposure needs to be taken, which means additional radiation to the patient.
The movement of the X-ray generator and the imaging device is traditionally synchronized so that the imaging device surface normal is perpendicular to the layer-of-interest. In this way the formed image is distorted as little as possible. A disadvantage of this approach is that the movement trajectory is quite complex. To achieve this motion, multiple motors are required (i.e. degrees of freedom) which also complicates the control electronics and algorithms, thus leading to higher cost. There are some imaging modalities in which the direction of radiation is intentionally not perpendicular to the surface normal, but the same drawbacks and advantages apply. In addition, one of the most severe issues experienced today in clinical applications of dental panoramic imaging is that the patient (object) does or cannot necessarily remain motionless for the whole duration of the scan (typically lasting 5 to 30 seconds). Even a small misalignment of the patient can result in that the part of the imaged layer being blurred or out of focus. In addition to panoramic images, a dentist may wish to create a transverse slice image of the patient's jaw. In transverse imaging, the layer-of-interest is perpendicular to the panoramic layer.
The existing commercially available, extra-oral (including, for example, panoramic and transverse) imaging solutions are either based on elongated (i.e., with an aspect ratio—length “m” divided by width “n”—of m/n=5 or more) time-delayed integration CCD sensors (not producing multiple frames) or on large-area 2D detectors with a computed tomography system backend where m/n is substantially equal to 1 (which, however, do produce multiple frames). The large-area 2D detectors are most often TFT panels and are particularly expensive because of the m/n.apprxeq.1 aspect ratio (approximately equal to one). The elongated detectors, which use a CCD coupled with a scintillator, apply the time-delayed integration (TDI) principle to form the image of the layer of interest. Time-Delay Integration is a method of synchronizing the shifting of the image signal captured in the pixel with the movement of the object image across the face of the CCD. This permits integration of more signal, increasing sensitivity, reducing noise and reducing image blur. According to this method, the integrated charges are clocked inside the detector logic (CCD) in the direction of the movement. Thus, at a given integration period ti, the charge for a fixed object volume v is integrated to a pixel pn. The object is moved so that the image of the plane-of-interest is moved (taking the magnification factor into account) exactly a pixel's width. After the integration period, the charges are transferred in such a way that if the image of the volume v is projected to pixel pi-1, the charged from pixel pi is transferred to pixel pi-1. The last pixel value in the row, which has no neighbor to which to transfer the charge, is read out and stored in the final image. In this way, the apparent integration time of an image pixel is the integration period multiplied by the width of the imaging device in pixels.
With the TDI principle, the clocking of the charges must be synchronized so that the apparent speed of the layer-of-interest in the imaging devices active in an integration period must be exactly the width of the pixel. If the speed is not matched, the image will appear blurred. A 2D flat image is formed from a single scan using an imaging device operating in the TDI mode. Multiple panoramic layers, transverse slicing or 3D imaging is not possible because only a single projection is saved.
In a dental cone beam computed tomography system (3-D imaging), multiple, non-TDI exposures are taken with a 2D area detector where m substantially equals n (i.e. within 20%). The movement is stopped before the exposure and the X-ray source is only active during this stationary period. The movement is continued after the exposure. In this manner, the movement doesn't have to be synchronized. Such systems require a higher dose to be administered to the patient and also longer examination times. The final image is formed as a layer calculated from the volumetric (3D) dataset constructed from the individual exposures or projections. The clear advantage of this method is that a full 3D volumetric dataset will be available after the procedure. However, with current solutions, the resolution of panoramic layer calculated from the 3D data is low compared to dedicated panoramic imaging systems (OPGs). Furthermore, the dose levels are much higher and maybe equally importantly the cost of such available systems is in the range of 200 kUSD-400 kUSD.
U.S. Pat. No. 6,496,557, entitled “Two Dimensional Slot X-ray Bone Densitometry, Radiography and Tomography,” describes a process in which multiple layers are formed by a so-called shift-and-add algorithm. The process described includes a system in which the motion of the imaging device is either linear or includes a rotational component around the focal point of the X-ray source. Unfortunately, such a system cannot be used in the field of dental X-ray imaging where the layer(s) of Interest run around or across the human jaw. Although such an approach may be useful in bone densitometry and in some other applications, it is in practice impossible to apply to dental panoramic or transverse imaging due to the fact that x-rays run essentially parallel to rather than across the layer-of-interest, should there be linear movement or rotation around the focal point. Additionally, the process disclosed in U.S. Pat. No. 6,496,557 would have another serious limitation if an attempt were to be made to apply it in the field of dental imaging. That limitation is the most serious issue in panoramic imaging and it results in partial image blurring due to misalignment of the patient or due to patient movement. Also, U.S. Pat. No. 6,496,557 fails to address the need of a system or a procedure that is able to simultaneously perform both panoramic as well as transverse imaging. Further, this patent fails to disclose a system operable in a dental imaging environment.
U.S. Pat. No. 5,784,429, entitled “Dental panoramic X-ray imaging apparatus,” describes a system in which multiple layers are calculated using plural tomographic images corresponding to plural tomographic planes which are arranged at predetermined intervals along the direction of the X-ray irradiation. A convolution process or a frequency process is conducted on a specific tomographic image by using image information of at least one of the tomographic images, so as to remove blur from the specific tomographic image. This patent describes a means of implementing different layers by using image intensifiers, CCD's or combinations thereof.
U.S. Pat. No. 7,136,452, entitled “Radiation Imaging System and Scanning Device,” discloses the use of frame mode CdTe-CMOS detectors for creating tomographic images in several applications including dental panoramic images using a dental X-ray setup. In U.S. Pat. No. 7,136,452, Spartiotis et al. disclose that the read-out speed or frame rate must be high enough to allow the detector to move by no more than half a pixel size or preferably even less per a read-out cycle. Emphasis added. However, if a system is implemented in such a way, serious limitations can arise such as the need to transfer a disproportionately large amount of data to a computer or the like in real-time. However, '452 patent is silent on the management of data to reconstruct and display an image in real-time, during the exposure. Furthermore, Spartiotis et al. teach nothing about one of the most serious issues facing dental radiology, that of partial image blurring due to patient misalignment and how to create automatically a focused layer. In such cases, part of the image (i.e., the panoramic layer-of-interest) is blurred while another part is in focus. Spartiotis et al. merely disclose that the frame rate needs be very high to collect frames in such a case in which the detector has moved by less than half a pixel, but does not teach how to accomplish this. Unfortunately, in this case, the amount of data produced during the exposure is unnecessarily high while at the same time, no material performance benefit is gained. Additionally, Spartiotis et al. do not suggest a means by which one might combine data to gain performance benefits and to correct blurring or to produce transverse image slices or even tomosynthentic 3D images.
Devices exist that are capable for example of performing transverse slicing or 3-D reconstructed images, but most often they require much longer X-ray scan times and non-continuous scan (i.e., a step by step scan). The long exposures are needed because the normally used digital imaging devices lack sensitivity and typically can “catch” only 1 out of 3 incoming x-rays. Further, non-continuous step-wise scans are necessary in the prior art because of the slow response and slow readout of the current rectangular or square flat panel TFT arrays. Higher doses and longer step-wise scans create higher risk and discomfort to the patient.
Additionally, the prior art does not suggest a means by which one might combine data to gain performance benefits and to correct blurring or to produce transverse image slices or even tomosynthentic 3D images. What is needed, therefore, is a system that limits the radiation doses a patient receives while maximizing the data output. Further, what is needed is a system which permits continuous fast, real-time X-ray scans, and a system and method of combining data from a single exposure to, not only reconstruct a panoramic layer of interest, but to also be able to correct part of the image that is blurred and furthermore produce transverse image slices and 3D images. Still further needed is a system that is capable of correcting blurring and which can produce transverse image slices or tomosynthentic 3D image.